Electrostatic discharge (ESD) is a well known phenomenon that must be taken into consideration during the design of many different types of circuits, such as processors, application specific integrated circuits, and the like. Essentially, when an ESD event occurs (typically in the form of a rapid pulse of undesired energy), the ESD circuit that is associated with the circuit is used to shunt the undesired energy away from the circuit to protect it.
An example of a known circuit that is used to provide ESD protection is illustrated in FIG. 1. As illustrates, there are two forward biased steering diodes Dn and Dp, where current from an ESD event that is negative will flow through Dn, whereas current from an ESD event that is positive will flow through Dp. With the current from an ESD event appropriately directed using the steering diodes, the current is then passed through a reverse biased sinker zener diode D1, which operates to provide a path to ground (sink) for the ESD pulse as a result of avalanche breakdown. The steering diodes Dn and Dp are choosen to minimize the effective capacitance of the entire ESD structure in this well known circuit topology illustrated in FIG. 1.
FIG. 2 illustrates a traditional layout of the steering diodes in the ESD circuit described in FIG. 1. In this traditional layout, at least one of the steering diodes (Dp in the example shown) is laid out in what is known as a fingered structure. This fingered structure has been used because it creates the maximum lateral area. As most of the current injection is lateral, the larger the lateral area, the more robust the structure. It has been found by the present inventor, however, that this fingered structure has the disadvantage of creating a voltage gradient which causes most of the ESD current to flow through a concentrated area 20 of the p-n junction of the steering diode Dp at each finger of the fingered layout structure as shown in FIG. 2. This concentration of current reduces the performance and reduces the reliability of the overall circuit.
Further, as voltages used in circuits generally have decreased, there has been a need for increasing ESD performance in the industry. However increasing ESD performance is associated with higher capacitance of the ESD device. For high speed signaling, increasing ESD capacitance is not acceptable, since it reduces the signal integrity of the system. Accordingly, there is a need for ESD circuits that can provide greater ESD protection with further reductions in input capacitance.